Passengers at O'Hare International Airport wait in a ticketing line May 13, 2014, in Chicago. Airlines individually determine the acceptable carry-on size, so don't let a false "FAA-approved luggage" claim determine what you buy.

The days of airlines offering free checked bags have gone the way of in-flight smoking and gateside farewells. So to save some cash, now is the time to invest in the best carry-on luggage that meets the rules for size and weight. Our buying guide looks at bag requirements common among the major airlines, as well as other features to consider when finding the best luggage deals.

Checked bag fees and carry-on luggage rules

With the exceptions of Southwest and JetBlue, all major airlines followed suit after American started checked bag fees in 2008. While airlines have made billions annually in checked luggage fees, budget-conscious travelers have been packing more carry-on bags—59 million additional pieces in 2010 alone, according to a TSA estimate.

Knowing the latest regulations and selecting the right carry-on can mean avoiding unexpected charges and travel headaches. Weight and size have a direct impact on the fees you will or won't pay, but there are other features to consider for your own convenience.

In response to slowed boarding times and consumer complaints about oversized bags, United Airlines launched a crackdown in March, installing bins where travelers can make sure their carry-ons are within the specified dimensions.

Passengers who don't make the cut at security must check the oversized bag for a $25 fee. Bags checked gateside will be free—a practice that's now common, given the rise in number of carry-ons and limited space. (Though most airlines don't have size guidelines for an additional personal item, United's new rules mandate 9" x 10" x 17" measurements.)

If you need the most packing space for your money, consider bags that have interior compression or that offer compatible organizing bags and systems to make packing more efficient.

Weight: does your carry-on bag need a slim-down?

Though a few US airlines have a carry-on weight limit of 40 pounds, many don't specify how heavy your bag can be. When traveling outside of the United States, keep in mind that airlines often have smaller weight limits for carry-ons—and these rules are enforced.

Irish airline Ryanair, for example, allows for only one carry-on weighing no more than 10 kg (about 22 pounds), so a heavy bag can cost you €50 (about $68) to check at the gate. Shifting excess weight to another suitcase might help, but there's a fee for overweight checked items as well.

To avoid additional baggage fees, consider a carry-on made with lightweight, durable materials, such as ballistic nylon, high-density polyester, and fiberglass. Lightweight carry-ons typically are classified as less than 10 pounds, though the lightest models weigh in at around three pounds. Lightweight carry-ons are available as backpacks or with hard shell exteriors as well.

Other features

In addition to the specific requirements for carry-on bags, there are some standard guidelines that apply to all luggage that you should consider. Here are some features to keep in mind. (For a complete guide on how to shop for the best luggage, click here.)

Soft shell versus hard shell

Carry-ons in high-tech fabrics are more resistant than ever to wear and tear, while hard shell luggage is now more lightweight and made with materials like polycarbonate, which is less likely to crack. Some hard shell designs even have easy-zip opening front pockets for last-minute items and electronics. Hard-shell carry-ons, however, have less give while being stowed onboard an aircraft.

Zip it up

Zippers are prone to break, and it's especially inconvenient when you're miles from home (or the ground). Opt for durable metal or plastic zipper chains versus coil zippers. Many bags also have self-repairing zippers, so travelers can simply move the zipper pull over an area where the teeth have separated to reseal. When shopping for a water-resistant carry-on in a hard or soft shell, be sure to check that the zipper area isn't a weak spot.

Spin your wheels

The latest in carry-on wheel technology: 360-degree spinner wheels, helping travelers avoid awkward bag topples while making turns and maneuvering through crowds. A four-wheeled spinner carry-on also allows travelers to turn the bag sideways for trips down the narrow airplane aisles while boarding and exiting. Because these wheels stick out more than the traditional rollers, they may break if mishandled; traditional roller bags offer recessed wheels with more protection.

Get a handle

Wheels aren't the only pivoting options for carry-ons. Some luggage companies also offer ergonomic handles in both one- and two-bar constructions. Carry-ons are also available with handles that are built to rotate to avoid hand and wrist strain.

Read the warranty

When investing in luggage, check for a warranty, and make sure you read the fine print. Even lifetime warranties often cover only manufacturing defects. Check to see if the warranty will cover additional damage. For example, in our warranty buying guide, we found Briggs and Riley Luggage will cost you extra, but offers a lifetime warranty, even for damage from baggage handlers. Manufacturer Victornox will even send you a temporary replacement while repairing certain lines of their luggage.